# Sort Colors

## June 11, 2020

## Introduction

Given an array with n objects colored red, white or blue, sort them in-place so that objects of the same color are adjacent, with the colors in the order red, white and blue.

Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively.

Note: You are not suppose to use the library’s sort function for this problem.

Example:

```
Input: [2,0,2,1,1,0]
Output: [0,0,1,1,2,2]
```

Follow up:

A rather straight forward solution is a two-pass algorithm using counting sort. * First, iterate the array counting number of 0’s, 1’s, and 2’s, then overwrite array with total number of 0’s, then 1’s and followed by 2’s. * Could you come up with a one-pass algorithm using only constant space?

## Solution

Let’s solve it in one loop

```
func sortColors(nums []int) {
n := len(nums)
for i, j, k := 0, n-1, 0; k <= j; {
switch nums[k] {
case 0:
if i < k {
nums[i], nums[k] = nums[k], nums[i]
i++
} else {
i++
k++
}
case 1:
k++
case 2:
nums[k], nums[j] = nums[j], nums[k]
j--
}
}
}
```

Performance of this solution is:

```
Runtime: 0 ms
Memory Usage: 2.1 MB
```